PDP is a display device using luminescence at cross section of matrix electrode which is ranged as length and width by plasma discharge of inert gas. A typical PDP is composed of two glass sheets, i.e., front and rear glass sheets having a thickness of 2.8˜3.0 mm which are sealed by glass frit. The indium tin oxide (ITO) as cathode is coated on an interior surface of the front glass sheet displaying images, and the Ni, Ag paste of anode and fluorescent material emitting red, green and blue colors are coated on an interior surface of the rear glass sheet.
In the beginning of PDP development, the size of a glass substrate was less than 20 inch, and soda lime glass widely used for building or automobile glass was commonly used. However, the more PDP development proceeds, the larger the PDP size becomes. Thus, such problems as size deformation by thermal expansion, and scratch during handling and processing became important.
Soda lime glass used for conventional glass substrate satisfied important properties for a glass substrate such as chemical stability, flatness, and optical properties. However, soda lime glass is not suitable for a glass substrate due to high thermal deformation rate and high content of Na2O. Specifically, the strain point of soda lime glass is around 510° C., and thus thermal deformation or decrease of productivity may be easily occurred during heat-treating process. Another problem of soda lime glass is to have reactivity with Ag electrode in PDP electrodes. The reaction may generate a colloid of Ag0 by ion permeation exchange between Ag+ and Na+ during heat treatment at a high temperature, and the generated colloid of Ag0 may absorb the wavelength of UV range to induce yellowing phenomenon. Therefore, soda lime glass had a disadvantage that the above reaction should be inhibited by coating SiO2 on a lower part of a substrate to prevent yellowing phenomenon. The reactivity between Ag+ and alkali metal can be anticipated by measuring volume resistivity. That is, high volume resistivity means that the diffusion rate of alkali metal is low, and thus the yellowing phenomenon can be prevented by increasing the volume resistivity of a glass substrate. The volume resistivity of conventional soda lime glass is measured as about 109 Ω·cm.
As described above, problems of conventional soda lime glass are the thermal deformation during heat treatment at a high temperature and the reactivity of Ag+ electrode and alkali metal. To solve these problems, glass composition for a PDP glass substrate having a higher strain point 60˜80° C. higher and a lower content of Na2O than conventional soda lime glass were developed. The strain point of glass may be increased by decreasing the content of alkali metal, with increasing the content of Al2O3. However, such change of components content inevitably induced increase of viscosity of glass at a high temperature, which result in a very unfavorable condition in a dissolution process of raw glass materials, a defoaming process of generated foam(fining process), and homogenization of glass melt. Therefore, there have been continuous efforts and new facility investments to solve these problems resulted from the increase of viscosity at a high temperature. The prior arts of glass composition for a substrate are explained in detail below.
Japanese Patent Laid-open Publication No. 03-40933 discloses a glass composition containing SiO2, Al2O3, alkali metal oxide, alkali earth metal oxide, and ZrO2. The composition is not deformed during heat treatment around 600° C., and has a similar thermal expansion coefficient to soda lime glass. However, this glass composition is very difficult to be practiced because the above Publication does not describe each specific amount of alkali metal oxide and alkali earth metal oxide, with only specifying total amounts. Also, the above composition further comprises Sb2O3 and As2O3 as fining agents to induce homogenization and defoaming. Excessive use of these components may cause corrosion of electrode or coloration of glass in an electric melting process.
Further, according to the Examples of the above Publication, the temperature is 1500° C. or more at the viscosity of glass melt of 102 poise. The fining process of removing foam in the glass melt is carried out when the viscosity of the glass melt is in the range of 10˜102 poise. Thus, as the above temperature is low, the fining process can be easily carried out. In other words, as the temperature corresponding to an area that the viscosity of glass melt is in the range of 10˜102 poise is low, the fining process can be easily carried out. And, the fining process of conventional soda lime glass melt is carried out in the temperature range of 1400˜1500° C., and the temperature of soda lime glass melt having the viscosity of 102 poise is about 1420° C.
In short, in the above Publication, the temperature is 1500° C. or more at the viscosity of the glass melt of 102 poise. This means that the fining process has to be carried out at a high temperature of greater than 100° C., compared with conventional soda lime glass. Thus, the composition of the above Publication has such disadvantages as increase of fuel cost and short life cycle of refractories. Further, if the defoaming process of removing foam generated in the melting furnace is carried out at a conventional process of producing soda lime glass, the decrease of product yield is inevitable due to fine foam, and thus an expensive complementary facility to supplement the decrease should be provided, which is another disadvantage of the composition of the Publication.
As an improved invention of the above described prior art, U.S. Pat. No. 5,599,754 mentions thermal expansion coefficient, transition temperature, and high temperature viscosity of a glass substrate composition, and discloses a process for producing glass by float method that is a flat glass production method. However, this process does not explain the strain point which is an essential property of a glass for PDP, and so the applicability is not sure. Also, in the Examples of this Patent, the temperature corresponding to the viscosity of glass melt of 102 poise is also 1500° C. or more. Thus, the composition also has such disadvantages as increase of fuel cost and short life cycle of refractories.
Also, Japanese Patent Laid-open Publication No. 08-133778 discloses a glass composition consisting of SiO2, alkali metal oxides, and alkali earth metal oxides. However, this glass composition does not comprise ZrO2, and thus the increase of the strain point of glass is not significant, the generation of devitrification cannot be prevented, and the water resistance and chemical resistance of glass cannot be increased. Further, the Examples of this Publication show that the temperature corresponding to the viscosity of glass melt of 102 poise is in the range of 1500˜1560° C. Thus, this composition also has a couple of disadvantages such as excessive melt load and cost loss thereby.
Further, Japanese Patent Laid-open Publication No. 2004-035295 discloses a composition comprising 1˜15% of MgO. However, this Publication does not provide data on the change of liquidus temperature according to the increase of MgO content, and a solution for defects according to the increase of devitrification tendency. Thus, the applicability of the composition is not sure. Therefore, production of a suitable glass substrate for high quality PDP seems to be difficult due to the devitrification defect which is inevitably accompanied in preparing flat glass.